Partial response signalling, also known as correlative coding, is a technique used in digital communication for high symbol rate packing, that is, for approaching the maximum utilization of a given bandwidth. This is particularly important for ultrahigh density digital recording. Such signalling introduces controlled intersymbol interference whereby the effect of the known interference can be removed. A particular class of partial response signalling is known as Class IV partial response signalling or modified duobinary signalling. It is also known as 1-D.sup.2 signalling as it utilizes a transversal filter wherein a second previous symbol is subtracted from the present symbol.
In Class IV partial response signalling it is conventional to provide precoding of binary data before recording. Such precoding involves changing the binary data to a precoded binary data stream whereby the 1-D.sup.2 signalling introduces controlled intersymbol interference in a manner that permits relatively simple decoding while decreasing the propagation of errors.
In digital tape recording of the sort in which a preferred embodiment of the present invention finds application, the digital binary data stream is precoded into a precoded binary data stream, which is recorded in binary form on magnetic tape. The data is precoded so that the decoding sections may decode transitions in ternary state as binary 1's. The data are later read from the tape by a pickup head which produces an analog signal that necessarily includes a certain amount of noise, mostly occasioned by the recording tape, the pickup head, and the preamplifier. The received analog data are then passed through a 1-D.sup.2 filter and sampled at particular intervals to provide controlled intersymbol interference. For playback data that were recorded at high linear packing density (approximately 45 KBPI), the Class IV partial response filter acts, to first order, as a matched filter for the record/playback process, resulting in an improved signal to noise ratio. This results in a ternary data stream in which there are three signal levels, commonly referred to as +1, 0, -1. The problem is that noise and changes in signal strength prevent the sampled signals from being exactly at each level. Changes in signal strength are readily eliminated by a form of normalization, sometimes referred to as envelope tracking. The effect of noise is reduced by detecting in accordance with some scheme. One common scheme in the prior art is simply to sample the analog ternary response appropriately and decode a bit as 0 if the absolute value is less than 0.5 and decode the bit as 1 if the absolute value is greater than 0.5. Other schemes involve what is known as maximum likelihood detection.
Such Class IV partial response signalling, precoding and maximum likelihood detection are explained at some length in the literature: P. Kabal et al., "Partial-Response Signalling," IEEE Transactions on Communications, Vol. Com-23, No. 9, September 1975, pp. 921-934; S. Pasupathy, "Correlative Coding, A Bandwidth-Efficient Signaling Scheme," IEEE Communications Society Magazine, July 1977, pp. 4-11; H. Kobayaski, "Correlative Level Coding and Maximum-Likelihood Decoding." IEEE Transactions on Information Theory, Vol. IT-17, No. 5, September 1971, pp. 586-594; G. D. Forney, Jr., "The Viterbi Algorithm," Proceedings of the IEEE, Vol. 61, No. 3, March 1973, pp. 268-278. An analog detection scheme is set forth in A. S. Acampora et al., "Analog Viterbi Decoding for High Speed Digital Satellite Channels," IEEE Transactions on Communications, Vol. Com-26, No. 10, October 1978, pp. 1463-1470.
In certain instances where the references refer to decoding, they refer to what is characterized herein as detecting, for a preferred embodiment of the present invention has been designed for use in a tape recording system as generally described in the patent application of Roger William Wood and Charles L. Matson filed this day for Encoder Verifier, in which patent application the component referred to as a decoder is another part of the system, the present invention being useful in the detection section of such system.